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Pentax XW 30,40 Focal Plane location

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#1 Simcal

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Posted 04 March 2021 - 10:09 AM

Hello,

would anyone have the measurements for the focal plane location, relative to the bottom of the barrel (or any landmark) for the latest Pentax SMC XW 30-R and 40-R eyepieces?

 

Thanks in advance!

simon.



#2 jgbackes

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Posted 04 March 2021 - 08:28 PM

Hello,

would anyone have the measurements for the focal plane location, relative to the bottom of the barrel (or any landmark) for the latest Pentax SMC XW 30-R and 40-R eyepieces?

 

Thanks in advance!

simon.

Simon, I own both, but I don't know where the "focal plane location" is. If you can give me a hint I'm happy to get out my calipers.

 

jeff



#3 Simcal

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Posted 04 March 2021 - 09:08 PM

Hi Jeff,

Thanks eh!  (sorry I maybe should have used the term field stop.. ?)

 

I have no idea where it is, I need a diagram like the one Televue provides.. (F = the field stop).

 

FieldStop


#4 Echolight

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Posted 04 March 2021 - 11:10 PM

I don't think there is a field stop other than the barrel itself on the XW40. But I'm often wrong.



#5 Simcal

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Posted 04 March 2021 - 11:55 PM

Am I correct in my understanding, that the Field Stop of the eyepiece must be placed at the telescope's Metal Back distance, to create focus?

 

-- If this assumption is wrong, then heck, I've wasted your time, sorry.

 

Background..I'm about to purchase a PreciseParts adapter for an FSQ-106ED,  which has MB=178mm, Focuser travel=30mm.  I'm tuning the optical path length to land the eyepiece(s) in the middle of travel. The TV DeLites are all -6.35mm (F above).  Thought I'd better check my Pentax eyepieces as well.



#6 sanbai

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Posted 05 March 2021 - 12:14 AM

Look here:
https://www.cloudyni...ax-xw-parfocal/

I googled "Pentax xw parfocal", this was the first thread in the search results. You may find more info around.
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#7 Ernest_SPB

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Posted 05 March 2021 - 03:55 AM

Front focal plane of the EPs (30, 40 mm XW Pentax) is places at connection of their body and setting barrel. In terms of TV, F = 0.


Edited by Ernest_SPB, 05 March 2021 - 03:56 AM.

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#8 Simcal

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Posted 05 March 2021 - 09:24 AM

Thanks everyone.  Sanbai, that linked topic provided me a solution.. I'll do a physical test with respect to the Delites and determine the difference in focus tube racking with my trusty vernier.  I've also sent Ricoh an email , and will share info if they provide it.


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#9 Starman1

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Posted 05 March 2021 - 04:58 PM

That info has been published for years: As Ernest said, the focal planes of all Pentax XW eyepieces is right at the shoulder where they stop when inserted.

That means, compared to the 30mm and 40mm, all the shorter focal lengths will focus inward, compered to the 30mm/40mm, by whatever the height of the 1.25" adapter is.

The shorter the adapter, the closer the 1.25" eyepieces will focus to the position the 30mm and 40mm focus.

I haven't checked it, but I bet the 1.25" ones would be very close to parfocal with the 30mm/40mm with an adapter like the TeleVue In-Travel adapter.


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#10 Simcal

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Posted 05 March 2021 - 11:43 PM

Thanks Don. Thanks Ernest. waytogo.gif



#11 Simcal

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Posted 09 March 2021 - 12:04 PM

Shout out to Bruce at Ricoh, for following up with me.   waytogo.gif bow.gif




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